Electrostatic sprayer system having a separate high resistivity conductor



1 M. A. R. POINT ETAL 3,157,255

ELECTROSTATIC SPRAYER SYSTEM HAVING A SEPARATE HIGH RESISTIVITYCONDUCTOR Filed May 7, 1962 lnvenlors M 14. R Poi/7i f? C. More/ UnitedStates Patent Ofiice 3,167,255 Patented Jan. 26, 1965 This inventionrelates to electrostatic sprayer systems, in which a sprayer assembly,such as a spray gun, comprises a nozzle having an electrode positionedin the path of divided spray particles of a material to be sprayed forcharging the particles individually to a high electric potential.

In recent years electrostatic spraying processes have been'increasinglyemployed for the purpose of coating articles with paint, plastic andother sprayable coating materials. particles must be charged to a veryhigh common DC. potential, of the orderof 100 kilovolts. Usually thesprayer is in the form of an at least partly metallic nozzle bodythrough which the spray particles are delivered and having a sharpannular edge or wire providing the electrode means which servesto'charge the spray particles to the desired high potential as theyissue from the nozzle. The sprayer body is connected to the high-voltageD.C. supply. In the use of such sprayers, the danger exists that themetallic sprayer body may be inadvertently brought close to. some nearbyconductive object at ground or nearground potential whereupon spark-overwill occur, and in view of the very high voltages involved suchsparkover is liable to damage the apparatus and expose personnel todiscomfort or injury.

To avoid this, it is usual to include a high protective resistance inseries with the high voltage supply, and such a resistance is generallyincorporated in the sprayer assembly itself. However, it is found thatsuch resistances are relatively short-lived when subjected to highcurrent surges. Moreover the presence of the large resistor in a spraygun near the forward end thereof tends to make the latter unwieldy.

It is an object of this invention to provide an improved electrostaticspray-coating system having enhanced sparkover protection. A furtherobject is to provide improved safety during electrostatic spray-coatingoperations, as well as to reduce the weight and size of electrostaticspray guns.

Basically the invention comprises an electrostatic spray coatingassembly having a high-voltage D.C. supply, a sprayer unit includingmeans for forming a spray, electrode means in the path of said spray andmeans connecting the electrode means to the supply for bringing theelectrode means to a high voltage potential to charge the sprayparticles, and including the improvement wherein said connecting meanscomprises a conductor having high distributed resistance over the lengththereof for reducing spark-over current in case said sprayer unit isinadvertently approached adjacent a conductive object at low potential.

An embodiment of the invention will now be described by way of examplewith reference to the accompanying drawings, wherein:

FIG. 1 schematically illustrates an electrostatic spray coating systemto which the invention is applied; and

FIG. 2 is an enlarged cross sectional view of a connector cable used insaid assembly.

The electrostatic spraying system shown in FIG. 1 generally comprises aspray gun 2 fitted with a nozzle 4 For such a process to be effectivethe spray which may be e.g. of the type described and claimed incopending application No. 186,436, filed April 10, 1962. The gun isshown as having flexible hose connections 6 and 8 to a source ofcompressed air and pulverulent plastic material respectively. The nozzle4 is at least partly metallic and has a sharp annular forward edge 10 toconstitute an ionizing electrode. Finger pressure on a control shown asa trigger 12 acts to discharge a jet of particles of plastic coatingmaterial suspended in air in an annular stream from the outlet of nozzle4, and simultaneously applies a high direct voltage, of say kilovolts ormore, to the nozzle body, by way of a flexible cable conductor 14connected to a high voltage D.C. supply source 16. Details of theconstruction and operation of the system may be found in theaforementioned patent application, and it will be here sufficient toindicate that the particles of the annular stream 18 as they move pastthe sharp annular electrode part 10 of the nozzle are substantially allcharged to a common high voltage throughout the thickness of the stream.The resulting charged cloud of particles is positively attracted to thesurface of an object 20 to be coated, which is generally grounded andsettle thereon and cling strongly to said surface by electrostaticattraction, with hardly any of the particles straying towards. othersurfaces.

In the operation of the'system, should the spray gun be inadvertentlyheld in a position where the nozzle 4 is 'too close to the surface ofthe object 20 or some other conductive surface at ground or near groundpotential, so that the high voltage difference between the nozzle andthe surface exceeds the spark-over potential of the intervening airpath, a high-energy arc will be struck with possibly damaging results.In accordance with this invention, this is averted by providing theconnector cable 14 with distributed resistance along its length. Thismay conveniently be achieved by using a wire of high resistivitymaterial, such as natural or synthetic rubber having pulverulentconductive material uniformly incorporated therein.

Thus, as shown in FIG. 2, the cable 14 may include a core 22 of ahigh-resistivity material as just described, having a linear specificresistance of the order of a few megohms per meter length, and a sheath26 of insulating material e.g. polyethylene. Preferably thehigh-resistivity core 22. is covered with an insulating tape 24 woundtherearound prior to applying the polyethylene sheath 26. The provisionof the tape insulation 24 is found advantageous in that it minimizes thetemperature rise occurring in the core on application of thepolyethylene sheath in an extruding or similar machine.

Extensive tests performed by the appiicants with electrostatic sprayingapparatus wherein the electrode voltage supply conductor was of thecharacter just described, have shown that the time constant introducedinto the circuit by the distributed resistance of the cable veryefiiciently dampens the current surge that would otherwise occur whenthe nozzle tip 4 is held adjacent to or in direct contact with aconductive object at ground potential; so that complete and foolproofsafety is obtained throughout electrostatic spraying operations. Theinvention eliminates the requirement for the provision of a largelocalized resistance at the tip of the spray gun, whereby the gun ismade more lightweight, and easier to wield, and its cost reduced.Experience has also shown that for a given degree of insulation adistributed-resistance cable according to the invention has higherstrength and longer life than a localized resistance, since at no pointalong the connecting conductor will there occur a concentration ofinstantaneously releasable energy. At the same time, owing to the verylow current values required in feeding the electrode means, the normaloperation of the electrostatic systems is not impaired by the presenceof the additional resistance. I

We are aware that distributed-resistance, e.g. high resistivity,conductors have been utilised in connection with internal combustionengine ignition systems for minimizing radio interferenceenergy'normally radiated from such systems. It is here noted that insuch prior arrangements the resistance thus introduced into the circuitsserved to dampen the R.-F. oscillatory energy continually present duringnormal operation of the system. In contrast, according to thisinvention, which is directed to unrelated art, thedistributed-resistance supply conductor is normally inoperative, butperforms its safety function of outstanding utility solely in case ofinadvertent manipulation of the electrostatic spray gun adjacent agrounded conductor.

It is to be understood that the specific embodiment of the inventionshown herein as applied to one form of electrostatic spray coatingsystem is illustrative only, and that other types of electrostatic spraycoating systems for the application of both liquid and solid dividedcoating materials will benefit similarly from the teachings of thisinvention.

It should be understood that in the ensuing claims the expressionsprayable composition is to be interpreted as designating any finelydispersable substance including both liquids and powders.

'We claim:

1. An electrostaticspray-coating system comprising in combination, asprayer unit having rear and forward ends, an external source of highvoltage direct current, and an external supply means for supplying asprayable composition, said sprayer unit having nozzle means including abody portion, a charging electrode at the forward end thereof, conduitmeans connecting the external supply means to said nozzle means in alocation to discharge a spray of sprayable composition past saidelectrode during operation of said sprayer unit, a separate highresistivity conductor having high stationary equally distributedresistance throughout the entire length thereof connected between saidexternal source of high voltage direct current, and said electrode forbringing said electrode to a high direct current ionizing potential,said supply means and said conductor separately connected to said nozzlemeans at the rear end thereof.

2. The electrostatic spray-coating system claimed in claim 1, whereinsaid conductor is a high resistivity material.

3. The electrostatic spray-coating system claimed in claim 1 whereinsaid conductor comprises a core of high resistivity rubber-like materialforming said conductor, insulating sheet material surrounding said core,and a sheath of polyethylene-base material surrounding said sheetmaterial.

References ited by the Examiner UNITED STATES PATENTS 2,714,623 8/55Wolcott 174-1201 2,790,053 4/57 Peterson 33866 2,861,163 11/58 Asakawa338-214 2,989,241 6/61 Badger 239-15 3,045,199 7/62 Shobert 3382143,105,218 9/63 Kozinski 338214 3,109,881 11/63 Publow 174-118 FOREIGNPATENTS 627,403 8/49 Great Britain.

865,763 4/61 Great Britain.

865,764 4/ 61 Great Britain.

865,765 4/ 61 Great Britain.

EVERETT W. KIRBY, Primary Examiner. MEYER PERLIN, LOUIS J. DEMBO,Examiners.

1. AN ELECTROSTATIC SPRAY-COATING SYSTEM COMPRISING IN COMBINATION ASPRAYER UNIT HAVING REAR AND FORWARD ENDS, AN EXTERNAL SOURCE OF HIGHVOLTAGE DIRECT CURRENT, AND AN EXTERNAL SUPPLY MEANS FOR SUPPLYING ASPRAYABLE COMPOSITION, SAID SPRAYER UNIT HAVING NOZZLE MEANS INCLUDING ABODY PORTION, A CHARGING ELECTRODE AT THE FORWARD END THEREOF, CONDUITMEANS CONNECTING THE EXTERNAL SUPPLY MEANS TO SAID NOZZLE MEANS IN ALOCATION TO DISCHARGE A SPRAY OF SYRAYABLE COMPOSITION PAST SAIDELECTRODE DURING OPERATION OF SAID SPRAYER UNIT, A SEPARATE HIGHRESISTIVITY CONDUCTOR HAVING HIGH STATIONARY EQUALLY DISTRIBUTEDRESISTANCE THROUGHOUT THE ENTIRE LENGTH THEREOF CONNECTED BETWEEN SAIDEXTERNAL SOURCE OF HIGH VOLTAGE DIRECT CURRENT, AND SAID ELECTRODE FORBRINGING SAID ELECTRODE TO A HIGH DIRECT CURRENT IONIZING POTENTIAL,SAID SUPPLY MEANS AND SAID CONDUCTOR SEPARATELY CONNECTED TO SAID NOZZLEMEANS AT THE REAR END THEREOF